The Address Resolution Protocol (ARP) is used to connect together the various networking layers within a computing network (e.g., to connect the Internet Protocol (IP) and Ethernet networking layers). Since networking hardware such as switches, hubs, routers, and bridges operate on, for example, Ethernet frames, these devices are unaware of the higher layer data carried by these frames. Similarly, IP layer devices, operating on IP packets, need to be able to transmit their IP data on the Ethernet layer. The ARP defines the mechanism by which IP capable hosts can exchange mappings of their Ethernet and IP addressing. The ARP is used primarily used in conjunction with IP Version 4 (IPv4).
More specifically, the ARP may be used to map IP addresses to a host address (i.e., a Media Access Control (MAC) address) that is recognized in the local network. For example, in IPv4, an IP address may be 32 bits long, but the Ethernet local area network (LAN) MAC addresses may be 48 bits long. A table, referred to herein as the ARP cache or ARP table, is used to maintain a correlation between each MAC address and its corresponding IP address.
The Neighbor Discovery Protocol (NDP) is another protocol in the Internet Protocol suite that is used in conjunction with Internet Protocol Version 6 (IPv6). The NDP is used for, among other tasks, discovery of other nodes and determined the addresses of the other nodes. In IPv6, the NDP is substantially the equivalent of the ARP used in IPv4.